1. Technical Field
This invention relates to an extremely-thin steel sheet which can adopt all temper grades of T1.about.T6 and DR8.about.DR10 and is suitable for use in various two-piece cans (SDC: Shallow-Drawn Can, DRDC: Drawn & Redrawn Can, DTRC: Drawn & Thin Redrawn Can, DWIC: Drawing & Wall Ironing Can) or three-piece cans (Side Seam Soldered Can, Side seam Welded Can, Thermoplastic Bonded Side Seam Can) and has uniform material properties and thickness accuracy in spite of extremely-thin thickness and wide-width and is excellent in economy as well as a method of producing the same.
In the invention, the term "extremely-thin steel sheet" means both of a blackplate for surface treatment and a surface treated steel sheet.
2. Background Art
The steel sheet for the can is subjected to various platings of Sn [including a tin plated steel having an Sn plated quantity of not less than 2.8 g/m.sup.2 and a thin tin plated steel sheet LTS (Lightly Tin Coated Steel) having an Sn plated quantity of less than 2.8 g/m.sup.2 ], Ni, Cr and the like and thereafter used in a drink can, a food can and the like.
The material property of the steel sheet for the can is defined by the temper grade. The temper grade is represented by a target value of Rockwell T hardness (HR30T), which is classified into T1-T6 in case of single-rolled products and into DR8-DR10 in case of double-rolled products represented by a target value of hardness (HR30T) and a target value of proof stress measured in a rolling direction.
Recently, the high-speed operation for can formation was progressed with the consumption of greater amount of drink cans and hence it became demanded to develop steel sheets suitable for the high-speed can formation. In the steel sheet for can, therefore, it was required to severely control not only the accuracy of the hardness but also dimensional precision and flatness of the steel sheet, lateral bending of steel strip and the like as compared with steel sheets for automobile.
On the other hand, a rationalization based on the reduction of can weight using a steel sheet of a thin thickness became recently a large tendency with the advance of the can formation technique even in can bodies such as 3-piece can and 2-piece can.
When the thickness is made thin, it is naturally impossible to avoid the lowering of the can strength. For this end, it is attempted to improve the can strength by changing a shape of a can through neck-in work, multistage neck-in work, smooth drastic neck-in work or the like or further to conduct the strengthening through deep drawing work, stretch work, bulging work, bottom doom work or the like after painting and baking.
In the production of 2-piece can, it tends to make the can height higher (i.e. increase of drawing ratio) for the increase of the content in addition to the can weight reduction.
From these recent situations, it is demanded to possess properties being conflicting in the conventional thought, which are high strength, very thin thickness, and excellent can formation workability and deep drawing workability as a steel sheet for the can. And also, in order to establish these properties, it is more important to improve the thickness precision and control the change of workability.
Furthermore, the painting of coil or lamination of film on the coil was recently put into practice, so that in order to efficiently conduct lamination operation for a body plate of, for example, 3-piece can, there was adopted a method of continuously laminating a film on a steel strip in a longitudinal direction and cutting out into a body plate per can unit through shearing or slitting. In this method, the film was laminated so as to render weld portion of the can body into the rolling direction (the height direction of the can is the rolling direction of the steel sheet), but in order to laminate the soft film at given set position with a high accuracy while rewinding the steel sheet, the demand of lateral bending accuracy and flatness of the steel strip became further severer. Because, when the film is laminated to the weld portion at a state of slightly shifting from the given set position, poor welding is caused to bring about large damage.
Thus, the lateral bending and flatness of the steel strip as a steel sheet for the can become demanded to be considerably excellent as compared with the conventional ones.
At the present, there is established a reasonable method for the can production in which approximately a full width of the steel strip for the can is rendered into a can except for few millimeters at a widthwise end portion, so that it is required that the material properties and thickness are uniform and dimensional precision such as tolerance of width and length, displacement of rectangle, lateral bending precision of steel strip or the like is excellent over the full width of the steel sheet for the can. Furthermore, the steel sheet having an excellent flatness is required for preventing the print displacement as previously mentioned. The ununiformity of the material properties largely exerts as a factor of the blackplate degrading the flatness, so that it is required to use an extremely-thin steel sheet having uniform material properties.
The uniformity of the thickness, particularly the uniformity of the thickness in the widthwise direction is important as mentioned above. In the conventional steel sheet for the can, the uniformity of the thickness was insufficient, so that when it was used in the production of the can, it was considered to design a large blank size so as to correspond a thickness of a raw material with the thickness result at the end portion in the widthwise direction being apt to be thinned in the punching out of circular blank to thereby provide a necessary can height. Therefore, the can height became unnecessarily higher in the widthwise central portion of the plate width being apt to be thickened to thereby decrease the yield, but also when the can body was taken off from a press machine, an upper portion of the can body was engaged with the press machine to prevent the removal from the machine and a new can body was charged in the machine before the removal and hence jamming phenomenon that plural can bodies were pressed several times was caused to largely damage the productivity.
In the 3-piece can, the can body was apt to become flat even if it was wound in form of a cylinder after flexor and hence the cylindrical body having a higher true circle was not obtained and there were problems that the thickness was locally thinner and the can strength was lacking even when using an extremely-thin and wide-width steel sheet for can having a high strength.
Furthermore, it is very important that the hardness in the widthwise direction of the steel strip is uniform. If a hard portion and a soft portion are mixedly existent in the widthwise direction of the steel strip, even when the rolling is carried out under the same rolling directions, the elongation of the soft portion is large and the elongation of the hard portion is small and hence the flatness becomes poor. Even if the poor flatness resulting from such a material property is apparently corrected by mechanical correction such as tension leveler or the like, when small blanks are subsequently formed by slit-cutting every can unit, local warping is again caused and hence there is caused a new problem that high-speed can formation becomes difficult.
Now, the conventional steel sheet for can was as narrow as 3 feet in an upper limit of formable width through a printing machine or coating machine, so that it was produced at a narrow width from the old time. However, when a line is newly arranged in accordance with the advance of the can formation method, the formation width became enlarged to not less than 4 feet (about 1220 mm) for the purpose of total rationalization from the production of steel sheet for can to finishing of can and high productivity. For this end, a wide-width steel strip having excellent productivity became demanded as a raw material for can.
As mentioned above, the thickness is extremely thin from a viewpoint of can weight reduction and the width is wider from a viewpoint of the productivity, so that it is newly required to totally use extremely-thin and wide-width steel sheets even in the field of the steel sheet for the can.
In the conventional technique, however, it was possible to merely produce the wide-width steel strip in view of the installation, but it was difficult to rationally correspond with the requirements as previously mentioned and there were, for example, problems that the thickness was thinned from the set value and the material properties were missed and the dimensional precision was poor. Particularly, these qualities were degraded in widthwise end portion and longitudinal end portion of the steel strip, so that there was a problem that these end portions were cut out and removed at the production step of the steel sheet to considerably lower the yield.
In the conventional technique, therefore, it was difficult to produce an extremely-thin and wide-width steel strip having uniform thickness and material properties over a full width of the steel sheet, and hence the size of the rationally producible steel strip was critical to be 0.20 mm in the thickness and about 950 mm in the width from a viewpoint of the sheet passing property in the continuous annealing (for example, described in "Brass and Tin-free Steel" (second edition), page 4 by Toyo Kohan Kabushiki Kaisha, published by Kabushiki Kaisha Agne). Even if steel strips having a width wider than the above were manufactured, it was difficult to provide substantially uniform thickness and material properties over not less than 95% of the width.
As a large factor obstructing the uniformity of the material properties, there are considered segregation of steel components and ununiformity of temperature in hot rolling and annealing. Among them, it can be said that the segregation of the steel components is substantially solved by the continuous casting and the annealing is solved by the advance of continuous annealing technique. Therefore, the remaining problem in the operation is considered to mainly lie in the hot rolling.
In the hot rolling, when using a hot rolling machine comprised of the conventional 4-stand rolling mill, there is no means for effectively controlling the plate crown, so that the variation of plate crown of about 100 .mu.m was caused by the change of roll profile with the lapse of time accompanied with thermal expansion and wearing of the work roll and the change of roll deflection deformation accompanied with the thickness and width of rolled material in a period ranging just from rearrangement of roll to next rearrangement.
In such a control of crown quantity was used 4 stage work roll shift, 6 stage HC roll or the like. In the extremely-thin and wide-width steel sheet, however, a variation of plate crown of not less than about 40 .mu.m was caused, so that the above control was insufficient from a viewpoint of ensuring the uniformity of material properties.
In any case, the conventional technique had a problem that the widthwise end portion and longitudinal end portion were cut out and removed by trimming operation or the like until the finish of a product as a steel sheet for can to largely lower the yield.
As mentioned above, it is strongly demanded to develop steel sheets for can having an excellent quality and being extremely thin and wider in width from a viewpoint of the reduction of production cost in the can body through can weight reduction and the improvement of productivity through width widening of coil.
However, when such a steel sheet was produced by the conventional production technique, there was a problem that the thickness and material properties (particularly hardness) were obliged to be ununiform in the widthwise direction. For this end, there were brought about not only the lowering of the yield through the trimming of widthwise end portion but also the lowering of high-speed passing property in the continuous annealing step, the lateral bending, the lowering of the flatness and the like. Furthermore, the lowering of product yield resulted from the poor shape of the can body and poor strength was brought about even in the production of the can body using such a steel sheet and hence new can forming method based on film laminated coil, coat coil or the like could not effectively be applied.
It is, therefore, an object of the invention to provide extremely-thin steel sheets for can having uniform material properties (particularly hardness) and uniform thickness in spite of extremely-thin and wide-width in light of the aforementioned problems of the conventional technique as well as a method of producing the same.
It is another object of the invention to provide an extremely-thin steel sheet for can capable of tempering to soft temper degree T1 or further harder temper grades T2-T6 and temper grade DR8-DR10 and being suitable for new can forming method and having uniform material properties (particularly hardness) and uniform thickness in spite of extremely-thin and wide-width as well as a method of producing the same.
Furthermore, a concrete object of the invention is to provide a high-quality, extremely-thin steel sheet having extremely-thin and wide-width of thickness: not more than 0.20 mm and width: not less than 950 mm and a thickness variation quantity within .+-.4% in a region other than both widthwise end portions of the steel sheet as cold rolled (provided that a ratio to width is not more than 5% in total of both side ends) and a hardness (HR30T) variation quantity within .+-.3.